Up to now, in a wavelength division multiplexing (WDM) transmission system, an erbium doped fiber amplifier (EDFA) is used with which wide spectrum bands may be amplified collectively.
In the transmission system using the EDFA, a degradation in an optical signal noise ratio (OSNR) which is caused by amplified spontaneous emission (ASE) light generated in the EDFA is one of bottlenecks in terms of a transmission performance.
Examples of an optical amplifier include a Raman amplifier in addition to the EDFA. The Raman amplifier has lower excitation light conversion efficiency than that of the EDFA, but the degradation in the OSNR is small. While a cost of an excitation laser diode (LD) is decreased, the Raman amplifier is widely used in recent years. In the Raman amplifier, to obtain a flat gain with respect to the wide spectrum bands, a technology of inputting excitation lights having plural different wavelengths in a transmission path is proposed.
Since a Raman gain in the Raman amplifier hardly relies on a signal wavelength number or a signal level, if an optical loss of the transmission path does not change, a flat gain wavelength characteristic is maintained by setting powers of the excitation lights having the plural wavelengths to be fixed.
See Japanese Laid-open Patent Publication No. 2001-7768 and Japanese Laid-open Patent Publication No. 10-73852.